Microstrip magnetic resonance (MR) coils are useful for magnetic resonance imaging (MRI) especially at high fields, because of their relative immunity to wavelength effects. These coils typically consist of a linear conducting strip backed by a dielectric layer and then a wider conducting shield layer. Microstrips are typically used for receiver coils and tuned to an integer multiple of half the wavelength (λ/2) at the Larmor frequency, and are oriented along the direction of the static magnetic field B0. Because of these length constraints, and given practical values for the dielectric layer, microstrips tend to stretch over fairly long distances (typically 30-50 cm). For this reason, groups of multiple microstrips are arrayed in the anterior/posterior (A/P) and/or right/left (R/L) directions. If parallel imaging is performed, then it is not possible to have significant accelerations in the superior/inferior (S/I) direction with these arrays.
In more particular details, microstrip receiver coils are typically designed as half-wave or quarter-wave resonators, which are aligned along the axis of the static magnetic field B0. In cylindrical-bore systems, this axis corresponds to the patient superior/inferior (S/I) direction. The relatively long wavelengths at imaging frequencies such as 63.8 MHz (1.5 T) or 127.7 MHz (3.0 T) therefore result in coils that are too long (e.g. 46 cm and 92 cm for quarter wave and half wave resonators) to be arrayed in the superior/inferior (S/I) direction, and so these receiver coils are typically only arrayed in the left-right and/or anterior/posterior directions. Although tuning with lumped elements has been considered for microstrips, the demonstrated designs are 30 cm or greater in length, which is too long for arraying in the S/I direction. This decreases their utility, because parallel imaging with accelerations in the S/I direction is precluded.